What makes sea level rise uneven

An illuminating article by Michael Lemonick just published in Yale Environment, which I summarise here, communicates some of the developing understanding of just how uneven sea level rise is likely to prove. It will vary greatly by region. There are a number of reasons for this. One is that the land is actually rising in some places, including northern Canada and Scandinavia, which are still recovering from the crushing weight of the Ice Age glaciers, albeit from 10,000 years ago. Their sea-level increases are less than the global average would suggest, since their land areas are rising a few millimeters a year. On the other hand land around the periphery of where the glaciers sat, such as Chesapeake Bay and the south of England, was squeezed upwards by the downward pressure nearby and has been sinking back by a few millimetres a year ever since, so sea level rise is greater than average in these regions. Land is also subsiding in coastal places where massive oil and gas extraction has occurred such as Louisiana.

A larger effect is from changes in prevailing winds, which can push water consistently toward the land or keep it at bay. The trade winds that blow west across the tropical Pacific, for example, boost average sea levels by as much as 24 inches on the western side of the ocean — in places such as the Philippines — compared with those in northern South America. If those winds shift with climate change, so would local sea levels.

Ocean currents also affect sea level rise. If the Gulf Stream were to slow, for example, that would force water to pile up behind what amounts to a partial blockage of the overturning current. That could force sea level along the U.S. coast to rise another 8 or so inches over the next century beyond the global average, given a medium-emissions scenario.

But the “gorilla in the room” according to Ronald Stouffer, of the U.S. Geophysical Fluid Dynamics Laboratory in Princeton NJ, is gravitation. The extra gravitational attraction of an undersea mountain range pulls water toward it, creating a literal, permanent bump on the surface of the sea, while the deficit of gravity near an undersea valley creates a depression in the water up above. A coastal mountain range pulls the water in its direction, raising sea level nearby. So do the massive icecaps that smother Greenland and Antarctica. They keep sea level higher than it would otherwise be for thousands of kilometers around both land masses, and correspondingly lower elsewhere.

If the polar ice sheets shrink, though – as they’re currently doing, especially in Greenland and West Antarctica – their gravitational pull weakens and so does their hold on the surrounding water. Their loss of mass not only contributes to overall sea level rise through meltwater but also allows some of the water held by their diminishing gravitational pull to go elsewhere – including the threatened east coast of the US. And it’s not a small effect. In Hawaii, for example, Stouffer estimates that a seven metre sea level rise caused by the disappearance of the Greenland ice sheet would have an extra two or three metres added to it. Whereas a beachfront property in Iceland would end up with more beach.

Jerry Mitrovica, a Harvard geophysicist who is working with Stouffer, comments that when he gives talks about this people don’t believe him. He doesn’t blame them. “It’s just wacky when you think about it, completely counterintuitive,” he says. “But it’s true.”

Mitrovica recalls that when he started looking at regional effects, some climate change deniers were noting that sea-level rise was happening at different rates in different regions, arguing that this proved there was no global trend, and thus no global warming. That was already a bogus argument, but now that he and others have begun investigating the gorilla in the living room, it’s even more absurd. The science is so straightforward, he says, that “if you saw that sea level was rising uniformly around the world, it would be proof that the big ice sheets are not melting.”

One wonders what the Christchurch City Council might make of all this. They’ve settled for planning for a 50 cm rise. “We’re following the Government’s advice and we’re not going out on a limb,” their spokesperson said primly. Apart from the fact that 50 cms is now inadequate advice for the century, the dynamics of regional variation suggest that an already complex set of considerations when planning for future sea level rise may have to be open to even more complication. Nick Smith will surely have to descend from his high horse: “The Government is not going to consider adjusting its policy every week.” Not that I’ve heard anyone asking for weekly adjustment – but annual reconsideration might be sensible.

Thanks. I did do some cursory searching for such information, but failed to locate anything. So I covered myself with "may be". But even within the global average for expected sea level rise the NZ government current advice looks to be well short of prudent.

If you go to the interactive wizardhttp://sealevel.colorado.edu/wizard.php
you can see which areas of the world are showing above or below average sea level rise over the last 2 decades. Levels do seem to be rising faster near the tropics.

Barry, this is actually a different phenomenon. The Mitrovica 'fingerprint' effect is due to the change in the shape of the geoid — the ideal equilibrium sea level in the absence of disturbances, an equipotential surface of the Earth's gravity field — due to the redistribution of masses as ice sheets melt. This picture OTOH shows the deviation of actual sea level from this ideal sea level, due to things like variations in temperature and salinity, ocean currents, air pressure changes etc.

And as you say, it's over the last 2 decades (the time when Topex-Poseidon type altimetric satellites have been up), so it's hard to say much on long-term trends from this. I would guess they are smaller than those due to the fingerprint effect.

NIWA sea level rise predictions were argued in an Environment Court case last year in Wellington concerning a new under-wharf car park: Waterfront Watch v Wellington Regional Council W43/2009.

Experts argued sea level rises by 2100 of between 0.5 and 1.1 metres. There was also discussion of combination conditions ie storm surge plus wave height producing 100 year storm events of 1.7 to 2.55 metres above current sea level.

The witness for Waterfront Watch, a reputable US expert, Dr Starley Thompson, based his analysis on the 2008 paper by Pffefer, which was based on newer information than the NIWA paper. Pffefer gives 0.8m as most likely by 2100 and 2m as possible but unlikely.

He made a couple of chilling additional comments.

On the return period for combination conditions:

"the joint return period of 100 years could in fact have a joint return period
substantially less than 10 years, perhaps as little as 1 to 2 years."

"As a rough estimate, I suggest that 0.08 metres of additional sea level rise per decade be added to any projection past the year 2100."

NIWA sea level rise predictions were argued in an Environment Court case last year in Wellington concerning a new under-wharf car park: Waterfront Watch v Wellington Regional Council W43/2009.

Experts argued sea level rises by 2100 of between 0.5 and 1.1 metres. There was also discussion of combination conditions ie storm surge plus wave height producing 100 year storm events of 1.7 to 2.55 metres above current sea level.

The witness for Waterfront Watch, a reputable US expert, Dr Starley Thompson, based his analysis on the 2008 paper by Pffefer, which was based on newer information than the NIWA paper. Pffefer gives 0.8m as most likely by 2100 and 2m as possible but unlikely.

He made a couple of chilling additional comments.

On the return period for combination conditions:

"the joint return period of 100 years could in fact have a joint return period
substantially less than 10 years, perhaps as little as 1 to 2 years."

"As a rough estimate, I suggest that 0.08 metres of additional sea level rise per decade be added to any projection past the year 2100."